The present invention relates to a portable telephone system such as a cordless telephone system constituted by a master unit and subsidiary units and a method of performing communication using portable telephone sets and, more particularly, to a portable telephone system capable of inter-subsidiary-unit speech communication mode and a method of performing communication using the same.
Recently, a portable telephone system constituted by a master unit connected to a public telephone line and subsidiary units connected to the master unit by radio has been provided. Some telephone systems are designed to allow direct speech communication between subsidiary units. In Japanese Patent Laid-Open No. 05-167523 or 05-102921, a radio section for inter-subsidiary-unit speech communication is provided for each subsidiary unit so that inter-subsidiary-unit speech communication can be realized by allowing each subsidiary unit to make a direct telephone call by using the radio section.
In such a conventional portable telephone system capable of inter-subsidiary-unit speech communication, in performing inter-subsidiary-unit speech communication, a telephone call is made to a remote subsidiary unit by radio. When the remote subsidiary unit is in a place where no radio electric field can reach the remote subsidiary unit, the remote subsidiary unit cannot be called, and direct radio speech communication cannot be performed either. In this case, when the remote subsidiary unit has a unique public line wire number, this line wire number is dialed to call the remote subsidiary unit by using the public line, thereby performing speech communication.
As described above, according to the inter-subsidiary-unit speech communication mode in the conventional portable telephone system, if a telephone call is made to a remote subsidiary unit by radio, and the telephone call fails, the public line wire number of the remote subsidiary unit must be dialed to perform a telephone call through the public line. For this reason, a calling operation is performed redundantly. In addition, with this operation, it takes much time to call the remote subsidiary unit.